Body mass and pressure

Aug 25th 2014

When studying Kevin Streelman's transition from backswing to downswing, why does the Swing Catalyst software show that most of the pressure is on his trail foot while most of his body mass is placed over his lead foot?

When analyzing the golf swing in the Swing Catalyst software, the seemingly independent actions of body mass and pressure can be confusing. Nowhere in the swing is this more apparent than in the transition from backswing to downswing, illustrated here with PGA Tour player Kevin Streelman. When studying this transition, why does the Swing Catalyst software show that most of the pressure is on the trail foot (R) while most of the body mass is placed over the lead foot (L)?

As explained in the article "Weight shift or pressure shift?", the Swing Catalyst Balance Plate measures the pressure applied to the ground by the golfer. Further, remember that pressure is force working over an area. This means that pressure is the result of the force exerted on the ground by the golfer. When standing still at address, pressure is usually distributed fairly evenly between the feet because the golfer's body mass (and therefore weight) is distributed fairly evenly between the feet (the Tour players we have studied tend to favor the lead foot slightly, typically displaying a 55-45 distribution). However, the positioning of body mass only determines the distribution of pressure in a passive situation, such as when standing still. During the swing, this relationship is not as clear. This is because the golfer can also actively push down, increasing pressure through force produced by the muscles.

Now consider the transition from backswing to downswing, the point where this confusion often arises. A golfer will push down with the trail foot to initiate the downswing. In the picture, this can be seen in the greater pressure on Kevin's right foot (R). The body mass, on the other hand, will start to move toward the lead foot in the transition. Notice that Kevin's body mass is favoring his lead foot (L), even though the pressure is favoring his trail foot (R). This is because he actively increases pressure on the trail foot (R) through muscle force. From this we can see that the distribution of pressure does not always follow the distribution of body mass. It will quickly "catch up" with the body mass in the downswing, though. Most good players employing a traditional swing style shift their pressure rapidly to the lead foot after the transition, illustrated here by the straight grey line in the center of pressure trace from Kevin's trail foot (R) to his lead foot (L).